Add TypeScript type declaration file

[Tooni]

• Adds a type declaration file, including types for the icons and their props (based on the declaration seen here)
• Adds code to generate.js to generate that type declaration file

This is a simple way to address https://github.com/Iconscout/react-unicons/issues/25, the TypeScript support issue.

The other PR addressing it is https://github.com/Iconscout/react-unicons/pull/50. It's great but is a full conversion. I thought perhaps the reason it wasn't merged is that it has too many changes.

My PR adds the types in a more minimal way, by adding solely the declaration file index.d.ts. This is enough to greatly improve the development experience for anyone using TypeScript i.e. most major tech companies.

Some screenshots of it working in VS Code:

[Tooni]

@rankarpan any chance this could get a review? who is maintaining this repository nowadays?

[jameshschuler]

Any way this PR can get some love?